Many Gram-negative pathogenic bacteria employ a complex protein secretion apparatus termed type III secretion system (T3SS) to transport bacterial effector proteins into eukaryotic host cytoplasm. The effector proteins delivered by T3SS are capable of modulating and interfering with host cellular processes, which then can cause diseases such as plague, typhoid fever and bacterial dysentery. More than 20 structural proteins, effector proteins, and chaperones are involved in assembly, function and regulation of this highly specialized molecular device. The translocon, one of the basic structural components of the T3SS, is a protein complex that is presumed to form a pore in the host cell membrane for translocation of effector proteins. The molecular mechanisms underlying protein translocation of the T3SS translocon remain poorly understood. Shigella flexneri is the etiologic agent of shigellosis, a global health problem that causes >1 million mortality worldwide annually. IpaB and IpaC have been identified as components of the translocon of S. flexneri. These proteins require significant conformational plasticity that is adaptive to their environments and functions. Our long-term goal is to understand the molecular mechanisms underlying the type III protein translocation using structural approaches. The proposed research will be focused on the three-dimensional structure of the IpaB:IpaC complex in the context of lipid membrane. The structural details about the translocon proteins and their interactions with the lipid membrane will be instrumental to understanding of T3SS, and will open avenues to new measures to control and prevent infection and diseases caused by this category of pathogenic microorganisms. Specific aims are to: (1) visualize the conformation of the IpaB:IpaC complex in the membrane-integrated form and the soluble form by means of electron cryo-microscopy;(2) initiate the crystallographic studies on the IpaB:IpgC complex.